The invention relates to heating apparatus comprising a flexible heating panel and electrical components for heating the panel. The invention is particularly applicable to electric blankets for domestic use, and will be specifically described in relation thereto. However, it will be appreciated that the invention may also be applied to many other forms of heated panels, such as under-carpet heaters, mattresses and pads.
Electric blankets commonly include a heating element in the form of inner and outer concentric coil conductors separated by an electrically insulating layer. One or other, or both, of the coils provide the heating effect. Should overheating occur, the insulating layer separating the two conductors is designed to melt, causing the two conductors to come into electrical contact with one another, causing a short circuit or low impedance path to occur. This causes a surge of current in the electric circuit containing the conductors, causing a fuse to blow and thus isolating the blanket from the mains supply.
There are many ways of detecting that a short circuit or low impedance path has developed between the two conductors, and examples of suitable circuits are described in British Patent Specifications Nos. 1155118, 1585921, 1599709, 2028608, 2266201 and 2286935.
However, existing systems typically require the use of at least 1200V diodes in series/parallel with the conductors of the heating element. Such diodes are both costly and prone to failure because of semi-conductor transient problems. Transient variations in the mains supply can, of course, be nullified by the use of a VDR device to prevent mains voltage peaks up to a maximum of, say, 800V (which is typical for a device connected to a 255V AC supply). However this still does not prevent the diodes occasionally breaking down in use of the blanket due, it is believed, to severe transient oscillations in the distributed inductance/capacitance network formed by the two coaxial windings.
Also, in many of the existing arrangements the four ends of the coaxial conductors are connected to an in-line switch or a bedside control unit for adjusting the heat setting of the blanket. This requires a multi-core cable to be connected to the blanket and such cables are bulky and heavy and relatively stiff, particularly under cold conditions, and accordingly may be difficult to manipulate and stow away. Also, fires may be caused by arcing due to failure of the multi-core electrical flex leading to the blanket. It is therefore desirable for the design of the electric blanket to be such that only a simple two-core flexible electrical cable is required between the blanket and the mains supply or control unit.
The invention is also applicable to low cost heating pads of smaller size than conventional electric blankets. Such heating pads are typically 30 cm.times.40 cm in dimensions and are commonly used to provide low cost personal heating in domestic situations. For example such pads may be stood on, sat on or placed around the sides of armchairs. Such heating pads are required to be fairly flexible and to have a fast warm-up. Consequently they generally incorporate a large length of heating element at close pitching, at a power/unit area rating which is typically ten times that for a conventional electric underblanket.
In presently available heating pads of this type the heating element is a single conducting wire wrapped on a rayon core and covered with a pvc sleeve. The conductor is disposed in a tortuous arrangement on the pad at a close pitching of the order of 12-15 mm. In order to provide overheating protection, one or more close differential thermostats are connected in the heating element, on the pad, and are arranged to break the circuit automatically in the event of the temperature of the element exceeding a predetermined value. However, since such heat pads are often subjected to considerable rucking or folding in use, this can lead to overheating of the element in areas away from the thermostats, with the result that the pad may overheat.
The present invention sets out to provide an electrically heated blanket, personal heating pad, or similar heating assembly in which the above-mentioned problems may be overcome.